The invention relates to a flame-retardant pressure equalizing valve according to the preamble of claim 1.
A valve of this type is for instance known as a commercially available product from Niikura Kogyo Co. of Yokohama, Japan. Such valves are for instance used in the shipping industry for equalizing undue excess pressures and reduced pressures in load tanks of tankers. Such excess pressures and reduced pressures occur inter alia during the filling and emptying of the tanks and in the case of temperature variations resulting from changing environmental conditions. An important requirement imposed on such valves according to the standard MSC/Circ. 373 of IMO Resolution A.519(13) (constituting an implementation rule of regulation II-1/59.1.5 of the SOLAS treaty 1974) is that when the valve is opened, a gas velocity of 30 m/s be immediately attained and that this velocity be maintained until the valve closes again. This prevents the flame from being able to penetrate into the tank if blown-off gases inflame. During the aeration of a reduced pressure in the tank, it is important that a strong minimum air flow into the tank be present, so that no inflammable gases can escape from the tank. Hence, it is very important that the valve open very promptly and close very promptly. Such valves are generally referred to as xe2x80x9chigh-velocity vantsxe2x80x9d.
With these known valves, a prompt opening is achieved utilizing the following principle. In closed condition, the force which, through relative excess pressure, is exerted on the valve body at the excess pressure side thereof, acts on a surface that is projected in opening direction and that is bounded by the circumferential edge of the first closure. When the relative excess pressure attains a value such that the closing force acting against the opening direction is exceeded, the valve body is displaced in opening direction and the valve opens along the first closure. Thus, the relative excess pressure also becomes active on the larger surface projected in opening direction and enclosed by the circumferential edge of the second closure. As a result, the force exerted on the valve body by the relative excess pressure suddenly increases, so that the valve is opened very promptly. As far as the prompt opening of the valve is concerned, the known valve is generally considered to be sufficiently effective.
A drawback of this known valve is that the valve body does not close at once, but tends to bounce up again during closing. This results in a temporarily intermittent gas flow of a low average velocity, which involves a substantial risk of a flame front backfiring via the valve. Further, the bouncing of the valve body causes considerable wear of the valve and an increased chance of leakage of the sealings along the closures.
Further, at given dimensions of the valve, a higher outlet velocity would be desirable, to enable highly inflammable gases to be brought, in a concentrated manner, at a greater distance from the valve and, accordingly, to further limit the danger of fire, explosion and poisoning, or, if the outlet velocity remains the same, to suffice with a more compact, lighter valve.
The object of the invention is to provide a pressure equalizing valve which, during closing, is less inclined to rebound and whereby, in comparison with existing valves, a higher outlet velocity of the gas is attained at the same relative excess pressure.
According to the present invention, this object is realized in that in a valve of the type indicated in the first paragraph, the second closure is spaced from the first closure in opening direction.
The fact that the valve according to the invention closes more properly, without rebound of the valve body, is attributed to the following effects.
During the closing of the valve, the measure in which the path of the still egressing gas changes on account of the valve body moving with the housing is relatively small, so that, when the valve closes, there is no or less increase of the force exerted on the valve body in opening direction.
Owing to the staggered position, in opening direction, of the first and the second closure relative to each other, the chamber thereinbetween has a greater length in opening direction. Thus, when the valve body is displaced through a particular distance, the relative volume change of the chamber between the first and the second closure is smaller than in the case of the known valve. This in turn results in that during the closing of the valve, which then, too, involves a still further decrease of the volume of the chamber between the first and second closures while the first and second sealings are already largely closed, 2 less abrupt increase of pressure occurs in this chamber than in the case of valves having first and second closures which, in opening direction, are located at the same level.
The relatively large volume of the air in the chamber between the first and the second closure further forms an accordingly large buffer providing that the velocity of the closing valve body is braked during the closing of the valve. As the chamber has a great length in opening direction, the valve body, after the closing of the second closure, can be braked very gradually over a large range, which prevents the valve body from rebounding. After the second closure has been closed again, the valve""s slow further closing does not result in a temporarily low velocity, because the valve is then already closed along the outer closing edge thereof, so that, except for slight leakage, if any, along the second closure on the outlet side, no gas egresses anymore from the valve.
In the valve according to the invention, the path along which the gas passes, during blowing off, through the valve has considerably fewer changes of direction than in the case of the known valve, as the radial displacement of the gas towards the opened second closure is accompanied by a displacement of that gas in opening direction which is substantially parallel to the flow direction of the passage. As a result, the resistance experienced by the gas flowing out through the opened valve is, in turn, relatively slight, so that high outlet velocities are realized. A favorable, vertical through-flow of the outflowing gas through a large distance is further promoted in that the flow of the egressing gas is little turbulent and can hence penetrate far into the environment. The limited turbulence in the gas flow is further advantageous, because the danger of a flame front backfiring through the valve is thus further limited.